Unfortunately with Backbone there's no control over the channels it uses for the sound effects, I think it tries to use a free channel if there is one and just cuts out a random channel when they're all in use, but it sounds fine with 3-channel mods.
Mr. Beanbag sounds good though, I wasn't sure if you had used 3 or 4 channel mods because I never notice the tune cutting out, good idea cutting out the base channel to play a sound effect.
I always wondered if a game could resume playing a note that was cut out, or if that channel would stay silent until the next note in that channel of the track is played, perhaps if the sample for the note was a long one or was inserted once with a loop. I also wonder about things like using the CPU to mix the channels to give you eight instead of four like how OctaMED does it, could it be used in an action game like a platformer or is there too much going on already?
Oh yeah, and I think Backbone is actually better suited to making a Metal Slug clone than any other type of game, so I think we should have a go at that too if people are interested.